1. Field of the Invention
This invention relates to the removal of contaminating materials from the earth. More particularly, the invention relates to locating and determining the composition of non-aqueous phase liquids (NAPLs) which may contaminate groundwater. The information gained from this invention is used in planning and executing remediation processes.
2. Description of Related Art
Contamination of groundwater has been found to be widespread and often very expensive to remedy. Health-threatening organic chemicals detected in groundwater have caused the greatest difficulties in remediation of contamination. Since the passage of environmental legislation in the United States in the 1980's, the expenditure of more than a billion dollars for site investigations and cleanup activities has advanced considerably the knowledge of the nature of organic contamination of groundwater (D. M. McKay and J. A. Cherry, Environ. Sci. Technol., Vol. 23, No. 6, 1989). Hundreds of plumes of organic contaminants have been delineated in the U.S. by networks of monitoring wells which are used to sample groundwater surrounding the wells for dissolved contaminants.
Because of the complexity of geologic formations and the fact that groundwater normally is not stationary but moves through soil or rock layers, it has been extremely difficult to determine the location of the free liquid contaminants which supply the dissolved materials contaminating groundwater. The dissolved organic contaminants in groundwater may have their origin in a spill or leak of organic liquid which occurred miles from the location of the contaminated water.
If organic liquids are lighter than water, they move downward through the vadose or partially air-saturated zone near the surface of the earth and accumulate on top of the water table. Such light non-aqueous phase liquids are referred to as "LNAPLs."The most commonly occurring contaminating LNAPL is motor fuel, such as gasoline. Motor fuels may contain small amounts of toxic chemicals such as benzene.
Some organic liquids lost near the surface of the earth have a higher density than water and continue to settle vertically through the water-saturated zone below the water table. Such liquids are called DNAPLs (dense non-aqueous phase liquids). Examples of DNAPLs are chlorinated hydrocarbons such as perchloroethylene and polychlorinated biphenyls, or PCBs. DNAPLs are particularly difficult to locate in the earth because they can continue to settle through the saturated zone until a low-permeability layer of soil or rock is reached, at which point they accumulate on top of the layer. If the low-permeability layer is of limited extent, the DNAPL may spill over from a higher to a lower low-permeability elevation. Thus, because of complex geology the route of DNAPLs moving downward through the earth is often not vertical. Also, there may be very limited information concerning the source of the contaminant.
The common method of remediation of groundwater contamination is simply to pump the contaminated water from the earth, to strip the contaminating chemicals from the water and to reinject or otherwise dispose of the water. Experience has taught that an initial decrease in contaminant concentrations in the water is followed by a leveling of concentration of contaminants. It is believed that the leveling of concentration of contaminants occurs because the water is continuing to be contaminated by dissolving free liquids present somewhere in the aquifer. Not knowing the size and location of NAPL pools and zones of residual NAPL makes it impossible to predict how long a pump-and-treat program must operate in order to clean the aquifer (McKay and Cherry, supra, 1989).
The solubility in water of some NAPLs which commonly occur in groundwater is as follows:
______________________________________ Benzene: 1780 mg/L Carbon tetrachloride: 785 Trichloroethylene 1100 ______________________________________
It is known that the solubility of organic materials in an aqueous solution can be increased far above these values by adding certain surfactants to water. Above a certain surfactant concentration, called the "critical micelle concentration," the surfactant solution contains micelles of surfactant molecules, which are aggregates of the surfactant molecules. The interior of the micelles, being hydrocarbon-like, can dissolve organic material or NAPLs to a much higher concentration than that due to the solubility of the NAPL in water. Increased solubility in solutions that are completely miscible with water can also be achieved by addition of cosolvents such as alcohols or ketones to water. Such solvent effects can also be used to increase the solubility of NAPLs to a concentration above the solubility in groundwater.
The extraction of organic pollutants using surfactant solutions has been tested (J. C. Fountain and D. S. Hodge, "Extraction of Organic Pollutants Using Enhanced Surfactant Flushing--Initial Field Test (Part 1)," New York State Center for Hazardous Waste Management, State University of New York at Buffalo, 1992). In the test, a surfactant solution was injected in some wells and produced from other wells. The concentration of a chlorinated hydrocarbon in the effluent surfactant solution was increased to more than 20 times its aqueous solubility. A 50-fold increase was observed in point samples taken during the test. Even higher increases could be achieved at higher surfactant concentrations. Other surfactants may solubilize certain NAPLs more effectively.
U.S. Pat. No. 4,997,313 discloses a method for use of an aqueous surfactant solution to remove organic contaminants from subsurface soil layers. The process is carried out continuously with the application of the surfactant solution to the surface of the earth and the removal of the solution containing the contaminant from a recovery well within the treatment area.
Although solubilizing solutions can be used to assist in removing contaminating NAPLs from soil or rock layers where the NAPLs may continue to dissolve in and contaminate large amounts of groundwater, there remains the problem of locating the undissolved NAPLs in the earth. It is known to drill wells into an aquifer and remove samples of the solids and liquids contained in the pores of the solids and analyze the liquids to determine the amount and composition of NAPLs. It is also known to produce fluids from the wells and to analyze the fluids for contaminating NAPLs, but if the NAPL is not present at high enough saturations in the pore space immediately around the well, only dissolved NAPL can be produced and the presence of liquid or undissolved NAPL will not be detected around the well. The NAPL can be present where the liquid fills only a fraction of the pore spaces of a geologic formation, up to about 20-35 per cent, depending on the size and structure of pores, and it will not flow through the formation because it is trapped in the pores by capillary forces. Ganglia of NAPL then are surrounded by water. But, at these saturations in the pore spaces there is sufficient NAPL to dissolve in and contaminate very large volumes of groundwater flowing through the formation. At liquid saturations above about 20-35 percent of the pore space, the NAPL will flow through the pore spaces and can be produced into a well to prove the presence of liquid around the well.
Many wells drilled in the search for free liquid NAPLs in the earth show no indication of free liquid--only dissolved NAPL. There is a long-felt need for a method of sampling a much larger volume of formation around these wells not having free liquid present at the well, to determine if free liquid is present in the vicinity of the well. Not only is there a need to determine if a NAPL is nearby a well, but there is a need to determine the likely composition of that liquid if it is present. Should the NAPL be composed of more than one component, knowledge of the identity and approximate mass fraction of each component is needed, so the environmental remediation engineer can design an effective treatment system. The location of NAPL and information on its composition can make practical the remediation of groundwater contamination which would otherwise exist as a long-term threat to health and a clean environment.